Vitamin B12 (cobalamin) is necessary for hematopoiesis and normal neuronal function. In humans, it is obtained only from animal proteins and requires intrinsic factor (IF) for absorption. The body uses its vitamin B12 stores very economically, reabsorbing vitamin B12 from the ileum and returning it to the liver; very little is excreted.

Vitamin B12 deficiency may be due to lack of IF secretion by gastric mucosa (eg, gastrectomy, gastric atrophy) or intestinal malabsorption (eg, ileal resection, small intestinal diseases).

Patients with serum B12 levels between 150 and 400 ng/L are considered borderline and should be evaluated further by functional tests for vitamin B12 deficiency. The plasma homocysteine level is a good screening test. A normal level effectively excludes vitamin B12 and folate deficiency in an asymptomatic patient. However, the test is not specific and many situations can cause an increased level. In contrast, an increased serum MMA level is more specific for cellular-level B12 deficiency and is not increased by folate deficiency.

Recent vitamin B12 administration could result in normal or elevated serum concentrations; therefore, this test should not be ordered on patients who have received a vitamin B12 injection within the past 2 weeks.

Many other conditions are known to cause an increase or decrease in the serum vitamin B12 concentration that should be considered in the interpretation of the assay results. As given below:

Increased Serum B12 level occurs in case of:

Ingestion of vitamin C

Ingestion of estrogens

Ingestion of vitamin A

Hepatocellular injury

Myeloproliferative disorder

Uremia

Decreased Serum B12 levels occurs in case of:

Pregnancy

Aspirin

Anticonvulsants

Colchicine

Ethanol ingestion

Contraceptive hormones

Smoking

Hemodialysis

Multiple myeloma

The evaluation of macrocytic anemia requires measurement of both vitamin B12 and folate levels; ideally, they should be measured simultaneously.

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